[clang] Allow installation of Clang .cfg files if they where generated. (PR #105580)

[Fangrui Song via cfe-commits]

https://github.com/MaskRay commented:

Apologies, I'm starting a 3-week vacation this Friday and will have limited availability. Happy for other reviewers to approve.

https://github.com/llvm/llvm-project/pull/105580